On 9/23/22 10:34, Marek Polacek wrote:
On Thu, Sep 22, 2022 at 06:14:44PM -0400, Jason Merrill wrote:
On 9/22/22 09:39, Marek Polacek wrote:
To improve compile times, the C++ library could use compiler built-ins
rather than implementing std::is_convertible (and _nothrow) as class
templates. This patch adds the built-ins. We already have
__is_constructible and __is_assignable, and the nothrow forms of those.
Microsoft (and clang, for compatibility) also provide an alias called
__is_convertible_to. I did not add it, but it would be trivial to do
so.
I noticed that our __is_assignable doesn't implement the "Access checks
are performed as if from a context unrelated to either type" requirement,
therefore std::is_assignable / __is_assignable give two different results
here:
class S {
operator int();
friend void g(); // #1
};
void
g ()
{
// #1 doesn't matter
static_assert(std::is_assignable<int&, S>::value, "");
static_assert(__is_assignable(int&, S), "");
}
This is not a problem if __is_assignable is not meant to be used by
the users.
That's fine, it's not.
Okay then. libstdc++ needs to make sure then that it's handled right.
This patch doesn't make libstdc++ use the new built-ins, but I had to
rename a class otherwise its name would clash with the new built-in.
Sigh, that's going to be a hassle when comparing compiler versions on
preprocessed code.
Yeah, I guess :/. Kind of like __integer_pack / __make_integer_seq.
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -3697,6 +3697,12 @@ diagnose_trait_expr (tree expr, tree args)
case CPTK_HAS_UNIQUE_OBJ_REPRESENTATIONS:
inform (loc, " %qT does not have unique object representations", t1);
break;
+ case CPTK_IS_CONVERTIBLE:
+ inform (loc, " %qT is not convertible from %qE", t2, t1);
+ break;
+ case CPTK_IS_NOTHROW_CONVERTIBLE:
+ inform (loc, " %qT is not %<nothrow%> convertible from %qE", t2, t1);
It's odd that the existing diagnostics quote "nothrow", which is not a
keyword. I wonder why these library traits didn't use "noexcept"?
Eh, yeah, only "throw" is. The quotes were deliberately added in
<https://gcc.gnu.org/pipermail/gcc-patches/2019-May/522333.html>. Should
I prepare a separate patch to use "%<noexcept%>" rather than "%<nothrow%>"?
OTOH, the traits have "nothrow" in their names, so maybe just go back to
"nothrow"?
The latter, I think. Or possibly "no-throw". I guess -Wformat-diag
wants "nothrow" quoted because of the attribute of that name.
--- a/gcc/cp/method.cc
+++ b/gcc/cp/method.cc
@@ -2236,6 +2236,37 @@ ref_xes_from_temporary (tree to, tree from, bool
direct_init_p)
return ref_conv_binds_directly (to, val, direct_init_p).is_false ();
}
+/* Return true if FROM can be converted to TO using implicit conversions,
+ or both FROM and TO are possibly cv-qualified void. NB: This doesn't
+ implement the "Access checks are performed as if from a context unrelated
+ to either type" restriction. */
+
+bool
+is_convertible (tree from, tree to)
You didn't want to add conversion to is*_xible?
No, it didn't look like a good fit. It does things we don't need, and
also has if VOID_TYPE_P -> return error_mark_node; which would be wrong
for __is_convertible.
I realized I'm not testing passing an incomplete type to the built-in,
but since that is UB, I reckon we don't need to test it (we issue
"error: invalid use of incomplete type").
But your patch does test that, in the existing call to check_trait_type
from finish_trait_expr?
The patch is OK.
Jason
